Polycrystalline silicon is used as a raw material for semiconductors, solar cells and the like, which are presently utilized in various fields and expected to be further developed and demanded in future, and therefore high-purity polycrystalline silicon is desired to be produced effectively. As a conventional production process of polycrystalline silicon, there has been, for example, the Siemens process in which a surface of a silicon rod placed inside a bell jar is heated and brought into contact with a source gas for silicon deposition containing a chlorosilane, such as trichlorosilane (SiHCl3; hereinafter referred to as TCS) and monosilane (SiH4), and a reducing gas such as hydrogen to deposit polycrystalline silicon.
The Siemens process is characterized in that high-purity silicon is obtained and is currently employed as the most common process. However, the Siemens process has a problem in that it is required to carry out very troublesome procedures such as installation of a silicon rod used as a seed, electrical heating, deposition, cooling, taking-out, and cleaning of a bell jar since the deposition is performed in a batch mode.
To overcome such a problem, the present applicant proposed, as a process and an apparatus for effectively producing silicon, a production process of polycrystalline silicon in which a source gas for silicon deposition is fed to a cylindrical reaction vessel heated to a temperature not lower than the melting point of silicon to deposit silicon in a molten state, and the molten silicon thus deposited is continuously dropped from the lower end of the reaction vessel and collected; and a production apparatus used in said process (see Patent Document 1).
Further, the present applicant proposed a production process of polycrystalline silicon in which, to a cylindrical reaction vessel heated to a temperature below the melting point of silicon, a source gas for silicon deposition is fed to deposit silicon, and thereafter, by heating the inner surface of the cylindrical reaction vessel to a temperature not lower than the melting point of silicon, a part or the whole of the deposited silicon is melted to drop and collect the deposited silicon; and a production apparatus used in said process (see Patent Document 2).
In such silicon production apparatuses, a carbon material such as graphite is usually used as a material for a cylindrical reaction vessel in which silicon is deposited.
However, if a reaction vessel made of a carbon material is used in such production processes silicon as described above, when silicon melt comes into contact with the carbon material, the inside of the carbon material is permeated with silicon and SiC (silicon carbide) is formed through reaction of silicon with carbon. Such permeation with silicon melt and formation of SiC cause a stress to the inside of the carbon material due to volumetric expansion associated with formation of SiC, and thereby the carbon material is cracked. Thus, there has been a problem of reducing production efficiency.
To overcome such a problem, there has been proposed a method in which the surface of the reaction vessel is coated with an SiC coating film having a thickness of 10 to 500 μm by CVD method (see Patent Document 3). However, in such a method of applying SiC-coating on the surface of the reaction vessel, the effect of preventing permeation with silicon melt is not sufficient and still cracking of the reaction vessel and the like occur resulting in stopping operation. Thus, the production efficiency has not been sufficiently improved yet.
Patent Document 1: JP-A-2002-29726
Patent Document 2: WO 2002/100777
Patent Document 3: JP-A-1997(H09)-157073
An object of the present invention is to provide a cylindrical vessel made of carbon whose inner surface comes into contact with silicon melt, wherein permeation with silicon melt is reduced, formation of SiC is suppressed, and the vessel is resistant to deformation even when volumetric expansion ascribable to silicon is brought about; and a production process of silicon using said vessel.
As a result of earnest studies to solve the above problems, the present inventors have found that, by using a specific carbon material, permeation with silicon melt is reduced, formation of SiC is suppressed and the vessel is resistant to deformation even with volumetric expansion ascribable to silicon, and further it is possible to reduce the amounts of carbon and impurities contained in a carbon material incorporated into the desired product, silicon, and they have completed the present invention.